Engine combustion chamber valves, such as intake and exhaust valves, are typically spring biased toward a valve closed position. In many internal combustion engines, the engine valves may be opened and closed by fixed profile cams in the engine, i.e., by a valve train element More specifically, valves may be opened or closed by one or more fixed lobes which may be an integral part of each of the cams. In some cases, the use of fixed profile cams may make it difficult to adjust the timings and/or amounts of engine valve lift. It may be desirable, however, to adjust valve opening times and/or lift for various engine operating conditions, such as positive power operation versus engine braking operation, or for different engine speeds during positive power and engine braking operation.
A method of adjusting valve timing and lift given a fixed cam profile, is to incorporate a “lost motion” device in the valve train linkage between the engine valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion dictated by a cam profile with a variable length mechanical, hydraulic, or other linkage means. The lost motion system may comprise a variable length device included in the valve train linkage between the cam and the engine valve. The lobe(s) on the cam may provide the “maximum” (longest dwell and greatest lift) motion needed for a range of engine operating conditions. When expanded fully, the variable length device (or lost motion system) may transmit all of the cam motion to the valve, and when contracted fully, transmit none or a reduced amount of cam motion to the valve. By selectively decreasing the length of the lost motion system, part or all of the motion imparted by the cam to the valve can be effectively subtracted or “lost”
Hydraulic-based lost motion systems may provide a variable length device through use of a hydraulically extendable and retractable piston assembly. The length of the device is shortened when the piston is retracted into its hydraulic chamber, and the length of the device is increased when the piston is extended out of the hydraulic chamber. Alternatively, a hydraulic-based lost motion system may utilize a hydraulic circuit including a master piston and a slave piston which is selectively charged with hydraulic fluid to actuate an engine valve. The master and slave circuit may be depleted of hydraulic fluid when it is desired to “lose” the valve actuation motion input to the master piston, and the circuit may be charged with hydraulic fluid when it is desired to transfer the motion from the master piston to the slave piston and the engine valve. One or more hydraulic fluid control valves may be used to control the flow of hydraulic fluid into and out of the hydraulic chamber or hydraulic circuit.
One type of lost motion system, known as a Variable Valve Actuation (VVA) system, may provide multiple levels of lost motion. Hydraulic VVA systems may employ a high-speed control valve, referred to herein as a trigger valve, to rapidly change the amount of hydraulic fluid in the hydraulic chamber or circuit between the master and slave lost motion pistons. The trigger valve may be capable of rapidly draining hydraulic fluid from the chamber or circuit, thereby allowing the lost motion system to selectively lose a portion of an engine valve event to provide variable levels of valve actuation.
In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may include a solenoid trigger valve in communication with the hydraulic circuit that includes the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit when the master piston is acted on by certain of the cam lobes. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the hydraulic fluid displaced by the motion of the master piston, which reciprocates in response to the cam lobe acting on it. When the solenoid is opened, the circuit may drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston and the engine valve.
Engine benefits from lost motion systems can be achieved by creating complex cam profiles with extra lobes or bumps to provide auxiliary valve lifts in addition to the conventional main intake and exhaust events. A number of unique modes of engine exhaust valve actuation may be produced by a lost motion system that includes multi-lobed cams. The lost motion system may be used to selectively cancel or activate different combinations of valve lifts made possible from the assortment of lobes provided on the exhaust cams. As a result, significant improvements may be made to both positive power and engine braking operation of the engine.
One particular engine valve actuation enabled by lost motion systems and methods operating in accordance with embodiments of the present invention is compression release engine braking operation. During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, an internal combustion engine into an air compressor. This air compressor effect may be accomplished by partially opening one or more exhaust valves near piston top dead center position for compression-release type braking, or by maintaining one or more exhaust valves in a partially open position for much or all of the piston motion for bleeder type braking. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted engine brake can develop retarding horsepower that is a substantial portion or even in excess of the level of operating horsepower developed by the engine in positive power.
Another engine valve actuation that may be provided using lost motion systems and methods in accordance with embodiments of the present invention is Early Exhaust Valve Opening (EEVO) during positive power operation of an engine. EEVO denotes the process of opening an engine exhaust valve for the exhaust stroke of the engine at a time that is earlier than the time the exhaust valve is normally opened for the exhaust stroke of the cylinder piston. Embodiments of the present invention contemplate selectively opening one or more engine exhaust valves for EEVO, starting as early as immediately after top dead center (TDC) position for the engine piston on the exhaust stroke, and as late as 90° crank angle degrees past TDC on the exhaust stroke. During positive power operation, EEVO operation may provide one or more benefits, including but not limited to: (i) providing combustion energy to the downstream exhaust path to enhance turbocharger speed and reduce turbo lag for improved transient performance and (ii) to provide higher torque production at lower engine speeds by increasing boost pressure, and (iii) sending exhaust energy (heat) to the engine after-treatment system to warm it up more quickly, (or maintain an elevated temperature), which may increase the efficiency of the after-treatment system and thus be able to reduce its required size and cost. Therefore there is a need for a valve actuation system that is capable of selectively providing EEVO in response to engine operation conditions.
System and method embodiments of the present invention recognize that the ability to provide both compression release engine braking and EEVO using engine exhaust valves would be advantageous. Accordingly, there is a need for a lost motion system, and in particular a variable valve actuation lost motion system, that utilizes a single control valve, preferably a trigger valve, for control of more than one engine valves to provide compression release engine braking and EEVO, and/or potentially other engine valve actuations.
Space and weight considerations are also of considerable concern to engine manufacturers. Accordingly it is desirable to reduce the size and weight of the engine subsystems responsible for valve actuation. Some embodiments of the present invention are directed towards meeting these needs by providing a simple on-off lost motion system for selectively providing EEVO during positive power engine operation and compression-release braking during engine braking operation.
Various embodiments of the present invention may meet one or more of the aforementioned needs and provide other benefits as well. Additional advantages of the invention are set forth, in part, in the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.